Fix handling of --skin-tone

When nargs=1 is given to argparse, it creates a list with one item, which is why
the additional [0] was needed when reading the value.
However, default='ask' means that the default value was 'ask' rather than
['ask'], causing the skin tone to be set to 'a', resulting in:
Traceback (most recent call last):
File "/usr/bin/rofimoji", line 1905, in <module>
emojis = compile_chosen_emojis(stdout.splitlines(), args.skin_tone[0])
File "/usr/bin/rofimoji", line 1870, in compile_chosen_emojis
emoji = select_skin_tone(emoji, skin_tone)
File "/usr/bin/rofimoji", line 1756, in select_skin_tone
return selected_emoji + fitzpatrick_modifiers_reversed[skin_tone]
KeyError: 'a'
Instead, don't pass nargs to argparse at all - with the default nargs value, we
just get the value (or default) back, so we don't need the [0] anymore.
Fixes #16